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1.
The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.  相似文献   

2.
Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west‐central New Hampshire, each major tectonic unit (nappe) displays a distinctive P–T path and metamorphic history that requires significant post‐metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low‐pressure, high‐temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. 40Ar/39Ar ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high‐level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages (c. 430–340 Ma). Both the metamorphic style and monazite ages suggest that the low‐grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257–300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian.  相似文献   

3.
The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite 40Ar/39Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D1 crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D2 event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D3 compressional exhumation event also evolved from south to north from 345 to 335 Ma. D1 to D3 deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D1, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.  相似文献   

4.
The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and 40Ar–39Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained 40Ar–39Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.  相似文献   

5.
Effect of metamorphic reactions on thermal evolution in collisional orogens   总被引：1，自引：0，他引：1  
The effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non‐reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism. This problem is typically resolved by invoking additional heat sources, such as high radiogenic heat production, elevated mantle heating or magmatism. Several models are tested that incorporate additional radiogenic heat sources; the elevated heating rates lead to stronger reaction and correspondingly larger thermal effects of metamorphism. The drop in peak temperatures may exceed 45 °C, the maximum temperature differences between the reacting and non‐reacting P–T–t paths may reach 60 °C, and pressure at peak temperature conditions is decreased by more than 0.2 GPa. Field observations suggest that devolatilization of metacarbonate rocks can also exert controls on metamorphic temperatures. Enthalpies were calculated for the reaction progress recorded by metacarbonate rocks in Vermont, and were used in models that include a layer of mixed metapelite–metacarbonate composition. A model with the average thermal structure and erosion rate of 1 mm year?1 can provide only half of the heat required to drive decarbonation reactions in a 10 km thick mid‐crustal layer containing 50 wt% of metacarbonate rock. Models with elevated heating rates, on the other hand, facilitated intensive devolatilization of the metacarbonate‐bearing layer. The reactions resulted in considerable changes in the model P–T–t paths and ～60 °C drop in metamorphic peak temperatures. Our results suggest that metamorphic reactions can play an important role in the thermal evolution of collisional settings and are likely to noticeably affect metamorphic P–T–t paths, peak metamorphic conditions and crustal geotherms. Decarbonation reactions in metacarbonate rocks may lead to even larger effects than those observed for metapelitic rocks. Endothermic effects of prograde reactions may be especially important in collisional settings containing additional heat sources and thus may pose further challenges for the ‘missing heat’ problem of Barrovian metamorphism.  相似文献   

6.
The Mahneshan Metamorphic Complex (MMC) is one of the Precambrian terrains exposed in the northwest of Iran. The MMC underwent two main phases of deformation (D1 and D2) and at least two metamorphic events (M1 and M2). Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite‐facies conditions. The dominant metamorphic mineral assemblage in metapelitic rocks (M1) is muscovite, biotite I, Garnet I, staurolite, Andalusite I and sillimanite. Peak metamorphism took place at 600–620°C and ∼7 kbar, corresponding to a depth of ca. 24 km. This was followed by decompression during exhumation of the crustal rocks up to the surface. The decrease of temperature and pressure during exhumation produced retrograde metamorphic assemblages (M2). Secondary phases such as garnet II biotite II, Andalusite II constrain the temperature and pressure of M2 retrograde metamorphism to 520–560°C and 2.5–3.5 kbar, respectively. The geothermal gradient obtained for the peak of metamorphism is 33°C km−1, which indicates that peak metamorphism was of Barrovian type and occurred under medium‐pressure conditions. The MMC followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal relaxation following tectonic thickening. The bulk chemistry of the MMC metapelites shows that their protoliths were deposited at an active continental margin. Together with the presence of palaeo‐suture zones and ophiolitic rocks around the high‐grade metamorphic rocks of the MMC, these features suggest that the Iranian Precambrian basement formed by an island‐arc type cratonization. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

7.
Here, we present results of the first 40Ar/39Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events.  相似文献   

8.
Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.  相似文献   

9.
《International Geology Review》2012,54(10):1270-1293
ABSTRACT

The Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable 40Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.  相似文献   

10.
During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.  相似文献   

11.
Sapphirine–quartz granulites from the Cocachacra region of the Arequipa Massif in southern Peru record early Neoproterozoic ultrahigh‐temperature metamorphism. Phase equilibrium modelling and zircon petrochronology are used to quantify timing and pressure–temperature (P–T) conditions of metamorphism. Modelling of three magnetite‐bearing sapphirine–quartz samples indicates peak temperatures of >950°C at ~0.7 GPa and a clockwise P–T evolution. Elevated concentrations of Al in orthopyroxene are also consistent with ultrahigh‐temperature conditions. Neoblastic zircon records ages of c. 1.0–0.9 Ga that are interpreted to record protracted ultrahigh‐temperature metamorphism. Th/U ratios of zircon of up to 100 reflect U‐depleted whole‐rock compositions. Concentrations of heavy rare earth elements in zircon do not show systematic trends with U–Pb age but do correlate with variable whole‐rock compositions. Very large positive Ce anomalies in zircon from two samples probably relate to strongly oxidizing conditions during neoblastic zircon crystallization. Low concentrations of Ti‐in‐zircon (<10 ppm) are interpreted to result from reduced titania activities due to the strongly oxidized nature of the granulites and the sequestration of titanium‐rich minerals away from the reaction volume. Whole‐rock compositions and oxidation state have a strong influence on the trace element composition of metamorphic zircon, which has implications for interpreting the geological significance of ages retrieved from zircon in oxidized metamorphic rocks.  相似文献   

12.
We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.  相似文献   

13.
Abstract 40Ar/39Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high-pressure, low- to medium-temperature metamorphism, M1, occurred about 50 Ma ago. Phengite has 40Ar* gradients that suggest that part of the scatter observed in conventional K–Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent 40Ar/39Ar plateaus at 15.0 ± 0.1 Ma, 11.8 ± 0.1 Ma, and 11.4 ± 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to M2. Toward lower M2 metamorphic grade, 40Ar/39Ar plateau ages of hornblendes increase to 19.8 ± 0.1 Ma; concomitantly the proportion of excess 40Ar in the spectra increases as well. We propose that the peak of M2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K–Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K–Ar ages from the same samples are in the range 12.1–13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M2 metamorphism. The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900–1500 MPa. Transition to normal crustal thicknesses ended the M1 metamorphism about 50 Ma ago. The M2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic ar°C in a southerly direction through the area.  相似文献   

14.
Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.  相似文献   

15.
High‐P/low‐T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560–550 Ma volcanic and sedimentary rocks and co‐magmatic plutonic units of the Caledonian terrane. The HMS consists of albite‐ and garnet‐porphyroblastic mica schist, with minor marble, calc‐silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc‐silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618–615 Ma, a minimum age for high‐P/low‐T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625–600 Ma subduction‐generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic‐Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now‐adjacent Brookville terrane.  相似文献   

16.
The Pinos terrane (Isle of Pines, W Cuba) is a coherent metamorphic complex that probably represents a portion of the continental margin of the Yucatan Block during the Mesozoic. Within the framework of other metamorphic terranes in the Greater Antilles, the Pinos terrane is characterized by the occurrence of high‐grade kyanite‐, sillimanite‐ and andalusite‐bearing metapelites and migmatites. Assessment and modelling of phase relations in these high grade rocks indicate that they reached a peak temperature of c. 750 °C at 11–12 kbar, and then underwent strong decompression to c. 3 kbar at c. 600 °C. Decompression was contemporaneous with the main synmetamorphic deformation in the area (D2), and was accompanied by segregation of trondhjemitic partial melts formed by wet melting of metapelites. Metamorphism terminated in the Uppermost Cretaceous (68 ± 2 Ma; 40Ar/39Ar dates on biotite and muscovite). The P–T–t‐deformation relations of the high‐grade rocks suggest that crustal thickening (during collision of this portion of the Yucatan margin with the Great Volcanic Arc of the Caribbean?) was followed by decompression interpreted to reflect exhumation by extension, possibly related to the initial development of the Yucatan Basin in the uppermost Cretaceous.  相似文献   

17.
Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near‐isothermal decompression at granulite facies temperatures in rocks from the Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite‐bearing assemblages, and secondary cordierite–sapphirine‐bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post‐peak metamorphic history is characterized by near‐isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post‐dated pervasive ductile deformation. K/Ar and 40Ar/39Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high‐temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and 40Ar/39Ar closure ages indicates that near‐isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one‐dimensional models for exhumation‐controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr?1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re‐equilibration.  相似文献   

18.
Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.  相似文献   

19.
Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.  相似文献   

20.
Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.  相似文献   

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